123d design can be used to model parts. 123d is an autodesk product and they have since introduced Fusion 360 which is a step between 123d and Inventor. It is free until you make 100k profit from using their software. Fusion 360 has the advantage of having a built in CAM program (HSM) so you can program the MPCNC with 1 program. If you don’t know much about modeling or cnc stuff then 123d and estlcam are probably the way to go.
You will want to build slightly over size say 32 inches if you have a 30 inch piece to cut.
Not sure how you account for the spacing difference by having 2 extruders. I am sure there is a firmware or slicer thing that you can set the distance between nozzles.